Pore systems and their correlation with oil enrichment in various lithofacies of saline lacustrine shale strata

Abstract

The potential of China's saline lacustrine shale oil resources is enormous. Currently, the storage space of saline lacustrine shale oil remains unclear, posing significant challenges for commercial development. In this study, we selected the samples from the typical saline lacustrine shale strata of the Lucaogou Formation, Jimusar Sag, Junggar Basin and investigated the pore systems and their relationship with oil content S₁ of different lithofacies. The pore structures were quantified by using the combination of low-pressure N₂ adsorption and mercury intrusion. TOC assessment, rock pyrolysis, and XRD were utilized for characterizing the organic geochemical and mineralogical parameters. The results showed that the organic matter comprising type III mainly appears in the clay-enriched lithofacies (types I₂, II₁ and III₂), while the organic matter types I, II₁ and II₂ can be found in other lithofacies. Regarding the nine lithofacies, siltstone has the highest oil content S₁, followed by the felsic-enriched lithofacies (types III₃ and III₁). The enrichment of oil in siltstone, carbonate rocks, and felsic-enriched lithofacies (types III₃ and III₁) is primarily attributed to macropores. Conversely, in clay-enriched lithofacies (types I₂, II₁ and III₂), the oil content S₁ is attributed to both the mesopore fractal dimensions (D₁ and D₂) and the TOC content. Moreover, the higher the complexity of the mesoporous structure (D₂) and the larger the macropore surface area in clayey carbonate felsic shale (II₃), the greater the oil content S₁. It is further observed that, macropores with size range between approximately 60 nm and 3000 nm are abundant in siltstone and felsic mineral-enriched lithofacies (i.e., carbonate felsic shale III₃ and felsic shale III₁). Furthermore, the siltstone shows the widest oil-rich macropore size range (range of 70–1000 nm), followed by felsic shale (III₁) (range of 150–1000) and carbonate felsic shale (III₃) (range of 100–110 nm). Siltstones and felsic-enriched shales are optimal for exploiting saline lacustrine shale deposits.